1. Field of the Invention
This invention relates to a shield excavator to be used for constructing a tunnel, a hole, a groove and the like having a polygonal, preferably, rectangular section.
2. Description of the Prior Art
It is desired that a rectangular shield excavator for constructing a tunnel, a hole, a groove and the like having a polygonal section be able to excavate beautifully without excavating each corner portion of the polygonal shape to look like an arc.
As one of such rectangular shield excavators, there is disclosed an excavator using a cutter assembly including a plurality of support rods extending in a first direction orthogonal to the axis of a quadrangularly tubular shield body and reciprocally movable in a second direction orthogonal to both of the first direction and the axis of the shield body, and a plurality of cutters mounted on each support rod, each cutter having a pair of bits, i.e. edges extending in the first direction, at the end the cutter in the second direction (Japanese Patent Appln. Public Disclosure (KOKAI) No. 1-310089). This excavator excavates a facing by reciprocating motion of each cutter with the reciprocating motion in the second direction of the cutter assembly. In this excavator, however, since the cutters are disposed such that the edges or bits of the adjacent cutters in the first direction are continuous, it only scrapes off the whole facing in such a manner as to plane wood. Therefore, the excavation efficiency is low.
As another rectangular shield excavator, there is disclosed an excavator using a cutter assembly composed of a drum rotatable about an axis extending in the direction orthogonal to the axis of a quadrangularly tubular shield body, and a plurality of cutters mounted on the peripheral surface of the drum (Japanese Patent Appln. Public Disclosure (KOKAI) No. 2-66295). This excavator excavates a facing by a rotational movement of the cutters with the rotational movement of the drum. In this known excavator, however, most of the forward end portion of the shield body is closed by the drum. As a result, the quantity of earth and sand received into the shield body is only a little and accordingly the excavation efficiency is low. Further, since in this known excavator the excavated earth and sand is gathered up on one side of the excavator by the rotational movement of the cutters, a great driving force is required.
As the other rectangular shield excavator, there is disclosed one using a cutter assembly composed of a plurality of arms angularly rotatable about an axis extending in a first direction orthogonal to the axis of the quadrangularly tubular shield body, the arms extending onward and backward, a support rod mounted on the forward end of the arms, the support rod extending in the first direction, and a plurality of cutters mounted on the support rod (U.S. Pat. No. 3,711,159). This excavator excavates a facing by a reciprocating motion of the cutters with a swinging motion of the arms and the support rods. In this excavator, however, since the cutter assembly is moved, pushing aside the excavated earth and sand filled so as to prevent a facing from collapsing, a great driving force is required for driving the cutter assembly.